This invention relates generally to semiconductor integrated circuits, and more particularly the invention relates to CMOS output circuits.
The standard CMOS output circuit as used in interfacing one of a plurality of devices to a common bus, for example, has an input signal applied to both gates of the CMOS transistor pair, and an inverted output is taken at the common terminal of the two serially-connected transistors. The output circuit can assume a high impedance state (i.e., tri-state) by taking the gate of the P-channel transistor high and the gate of the N-channel transistor low. However, since different peripheral devices tied to common bus can have different operating voltage levels, which are selectively applied to the bus, an output circuit can have a voltage applied to its output terminal which is greater in magnitude than the supply voltages for that output circuit. In this circumstance, the substrate/source-drain diodes of the transistors can forward bias thus loading the common bus and causing potential latchup problems.